The present invention relates generally to a pipe switch for use in a pneumatic conveying unit used for transporting pulverized or granular material. The pipe switch is of the type having a housing delimiting together with at least cover a cylindrical inner space communicating with a feeding connection and a plurality of discharge connections. A rotary plug is contained in the inner space and is formed with a plurality of channels corresponding in number to the discharge connections. Control means are coupled to the plug to rotate the same into a plurality of switching positions in which during the transport of the pulverized or granular material the feeding connection is connected to a selected discharge connection. Air gap between the housing and the plug is sealed both at the inlet openings and at the outlet openings of the channels by means of sealing webs.
Pipe switches of this kind, which are used mainly for switching over the streams of pulverized or granular material, must be constructed with a certain play between the rotary plug and the stationary housing in order to obtain sufficient mobility of the plug. This play is achieved by an air gap between the plug and the housing in the range of one or more tenths of a millimeter. Such a minute play requires a considerable accuracy in manufacturing the plugs and the housings and perfectly cylindrical surfaces in the jacket of the plug and the inner wall of the housing. Such a high degree of accuracy, however, requires time-consuming and expensive manufacturing procedures.
If the air gap is not sealed, the deposits of dust and minute particles of material build up and clog the gap. As a consequence, the position-adjusting movement of the plug can be impaired or even blocked, particularly when thermal expansion occurs. Such prior-art switches without sealing means is known for example from the DD Pat. No. 27 714.
In another known pipe switch (German published patent application No. 1 916, 581 and German publication No. 1 555 057) annular grooves are formed in the inner wall of the housing around the mouth of respective feeding and discharging connections and sealing means in the form of blowable hose sections are inserted into these grooves for being blown up by pressure air and during the switching of the plug they are pressure-relieved. This kind of pipe switch therefore cannot be operated in the conveyed stream or during the conveying operation, and during the position-adjusting movement of the plug the edges of the channels of the latter override the sealing means so that the sealing hoses are mechanically strained and subject to additional wear.
Another pipe switch is known from prior-art, for instance from German published patent application Nos. 2 051 177 and 2 060 803 in which the plug is provided with one or more sealing pistons which are actuated mechanically, electromagnetically, pneumatically or hydraulically for sealing the unused connections in the housing. The sealing pistons during the position-adjusting movement of the plug are withdrawn from the inner wall of the housing in the counter-direction to its pressing force. Even this known pipe switch can be switched over only upon the interruption of the conveying process.
From the prior art it is also known a pipe switch (German published patent application No. 2 148 557) having a switchable plug provided with a curved channel the end openings of which are sealed by sealing means inserted in a recess in the plug body and surrounding the individual channel openings. The sealing means are in the form of blowable elastic hoses and in its blown up condition closely engage both the plug around the channel and the inner wall of the housing. Similarly as in the preceding examples, in this known embodiment of pipe switch the pressurized sealing hoses must be pressure-relieved in order to perform the switching movement, and the conveying stream of material must be interrupted.